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YFe2Al10 forms in a well-ordered crystal structure, with a unique site for the magnetic atom Fe. At elevated temperatures YFe2Al10 behaves electronically as a good metal. At low temperatures on the other hand, signatures of correlated behavior develop and eventually non-Fermi-liquid scaling dominates all of the physical properties: the electrical resistivity develops a negative temperature coefficient below 20 K, and the specific heat Cp(T)/T and χ(T) both assume a power-law increase upon lowering the temperature below ≃3 K. Lowest temperature studies have failed to find magnetic ordering in YFe2Al10 in spite of compelling evidence that there is impending order in this compound and that it is indeed of ferromagnetic character. Here we report on measurements of thermal and electronic transport as well as heat capacity studies on the series of compounds YFe2 + δAl10 with δ ≤ |0.1|, with the purpose of testing the stability of the low-temperature divergences in physical properties against off-stoichiometric Fe.

Arsenic Makes a Difference A quantum critical point (QCP) occurs when quantum fluctuations, which do not go away even at absolute zero, cause a gradual (so-called second order) phase change. QCPs have been observed in ferromagnets, but for ferromagnetic metals, the evidence is less clear-cut and it is thought that, as the temperature is lowered, another order—such as superconductivity—will prevent the formation of a QCP. However, Steppke et al. (p. 933 ), using specific heat and magnetic susceptibility measurements, found strong evidence for a QCP in a quasi–one-dimensional heavy fermion material, YbNi 4 (P 1− x As x ) 2 , near an Arsenic substitution level of about 10%. The results present a challenge to theories about quantum criticality in ferromagnets.

Ferromagnetic correlations are discussed on the basis of shift K4f and spin–lattice-relaxation (SLR) rate R4f(T) = (1/T1T)4f investigations for YbNi4P2 and YbRh2Si2. For YbRh2Si2 the coexistence of antiferromagnetic (afm) and ferromagnetic (fm) fluctuations at the edge of magnetic order is strongly related to the non-Fermi-liquid behaviour (NFL). The analysis of the NMR results for YbNi4P2 in the modified Korringa framework for correlated local moment metals yields the enhancement factor (α) which is a measure of the q -space dependence of the complex dynamic susceptibility. Surprisingly the (α) behavior of YbNi4P2is very similar to YbRh2Si2and evidences strongly the evolution of an afm (q ≠ 0) component in the q = 0 dominated complex dynamic susceptibility χ’’ (q,ωn,T) at low temperatures and in small fields. This suggests the presence of ferro- and antiferromagnetic fluctuations at the QCP and the freezing out of the afm ones by applying magnetic fields and tuning towards FL state. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we present a comprehensive study of heat capacity, resistivity, neutron diffraction and magnetization measurements on Yb(Rh0.42Co0.58)2Si2. Heat capacity and resistivity measurements revealed one phase transition at Tm ≈ 820 mK into the magnetically ordered phase, but in neutron diffraction experiments the magnetic intensity at the commensurate position vanished already at a lower TL ≈ 700 mK. From magnetization measurements we could derive the existence of two magnetic phases, an antiferromagnetic phase below TL ≈ 700 mK and an intermediate phase in the temperature range , which shows surprisingly ferromagnetic properties when applying magnetic fields along the [001] direction.

We investigate the influence of small amounts of Ir and La substitution on the low-temperature thermal conductivity κ and thermopower S of YbRh2Si2. While the thermal conductivity is reduced in both cases due to the introduction of additional impurities, the effect on the thermopower is non-uniform: Ir substitution leads to a reduction of the absolute value of the thermopower. This is attributed largely to an enhanced disorder scattering. On the other hand, La substitution increases the absolute thermopower values. We discuss the possibility that this is due to band structure modifications upon doping into the Yb sublattice.

We present investigations on the magnetic susceptibility χ(T), the specific heat coefficient γ(T) = C(T)/T, and the spin-lattice relaxation (SLR) rate on the two isostructural iron aluminides YFe2Al10 and YbFe2Al10. Below 10 K, both systems display a monotonous field-dependent increase with decreasing temperature in χ(T), γ(T) and the SLR coefficient 27R(T) = 27(1/T1T). These divergences are associated with spin fluctuations near a quantum critical point (QCP). Most probably, the QCP is of ferromagnetic (FM) nature, but so far no magnetic order is observed in either system down to 0.35 K. The application of moderate fields of a few Tesla suppresses the low-T upturn in the properties. The comparison between zero-field and high field measurements allow for the determination of enhancement factors in χ(T), the γ(T), and the spin-lattice relaxation coefficient (SLRC) 27R(T). Surprisingly, the relative enhancement in γ(T) and the T-dependence [∝ ln(1/T)] are very similar for both systems. In the SLRC, the enhancement is larger for the Yb-system but both show ln(1/T) behavior for T 0. The divergences are very similar because the Yb ions in YbFe2Al10 are in an intermediate valence state at low temperatures and contribute only very little towards the observed spin fluctuations. This indicates strongly the same underlying physics and the same energy scales for both systems. We propose the presence of dominant q = 0 low-energy FM spin fluctuations from weak itinerant Fe as the origin for the observed logarithmic divergences.

The site-diluted compound (Yb$_{1-x}$Lu$_x$)$_4$As$_3$ is a scarce realization of the linear Heisenberg antiferromagnet partitioned into finite-size segments and is an ideal model compound for studying field-dependent effects of quenched disorder in the one-dimensional antiferromagnets. It differentiates from the systems studied so far in two aspects - the type of randomness and the nature of the energy gap in the pure sample. We have measured the specific heat of single-crystal (Yb$_{1-x}$Lu$_x$)$_4$As$_3$ in magnetic fields up to 19.5 T. The contribution $C_{\perp}$ arising from the magnetic subsystem in an applied magnetic field perpendicular to the chains is determined. Compared to pure Yb$_4$As$_3$, for which $C_{\perp}$ indicates a gap opening, for diluted systems a non-exponential decay is found at low temperatures which is consistent with the thermodynamic scaling of the specific heat established for a Bose-glass phase.

We address the origin of the magnetic-field-independent $\ensuremath{-}|A|{T}^{1/2}$ term observed in the low-temperature resistivity of several As-based metallic systems of the PbFCl structure type. For the layered compound ${\mathrm{ZrAs}}_{1.58}{\mathrm{Se}}_{0.39}$, we show that vacancies in the square nets of As give rise to the low-temperature transport anomaly over a wide temperature regime of almost two decades in temperature. This low-temperature behavior is in line with the nonmagnetic version of the two-channel Kondo effect, whose origin we ascribe to a dynamic Jahn-Teller effect operating at the vacancy-carrying As layer with a ${C}_{4}$ symmetry. The pair-breaking nature of the dynamical defects in the square nets of As explains the low superconducting transition temperature ${T}_{\mathrm{c}}\ensuremath{\approx}0.14\text{ }\text{ }\mathrm{K}$ of ${\mathrm{ZrAs}}_{1.58}{\mathrm{Se}}_{0.39}$ compared to the free-of-vacancies homologue ${\mathrm{ZrP}}_{1.54}{\mathrm{S}}_{0.46}$ (${T}_{\mathrm{c}}\ensuremath{\approx}3.7\text{ }\text{ }\mathrm{K}$). Our findings should be relevant to a wide class of metals with disordered pnictogen layers.

We investigated the superconducting order parameter of the filled skutterudite ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$, with a transition temperature of ${T}_{c}=8.3$ K. To this end, we performed temperature and magnetic-field dependent specific-heat and thermal-conductivity measurements. All data are compatible with a single superconducting $s$-wave gap. However, a multiband scenario cannot be ruled out. The results are discussed in the context of previous studies on the substitution series ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Pt}}_{4}{\mathrm{Ge}}_{12}$. They suggest compatible order parameters for the two end compounds ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$ and ${\mathrm{PrPt}}_{4}{\mathrm{Ge}}_{12}$. This is not consistent with a single $s$-wave gap in ${\mathrm{LaPt}}_{4}{\mathrm{Ge}}_{12}$ considering previous reports of unconventional and/or multiband superconductivity in ${\mathrm{PrPt}}_{4}{\mathrm{Ge}}_{12}$.

Nature Reviews Materials 1, 16051 (2016) In the ‘Emerging quantum criticality in Kondo lattices’ section on page 10, the value of the crystal-field splitting was incorrectly stated as 0.1 eV. The correct value is 0.01 eV. This has been corrected in both the HTML and PDF versions.

This paper presents the results of specific-heat and magnetization measurements, in particular their field-orientation dependence, on the first discovered heavy-fermion superconductor ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ (${T}_{\mathrm{c}}\ensuremath{\sim}0.6$ K). We discuss the superconducting gap structure and the origin of the anomalous pair-breaking phenomena, leading, e.g., to the suppression of the upper critical field ${H}_{\mathrm{c}2}$, found in the high-field region. The data show that the anomalous pair breaking becomes prominent below about 0.15 K in any field direction, but occurs closer to ${H}_{\mathrm{c}2}$ for $H\ensuremath{\parallel}c$. The presence of this anomaly is confirmed by the fact that the specific-heat and magnetization data satisfy standard thermodynamic relations. Concerning the gap structure, field-angle dependencies of the low-temperature specific heat within the $ab$ and $ac$ planes do not show any evidence for gap nodes. From microscopic calculations in the framework of a two-band full-gap model, the power-law-like temperature dependencies of $C$ and $1/{T}_{1}$, reminiscent of nodal superconductivity, have been reproduced reasonably. These facts further support multiband full-gap superconductivity in ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$.

We report on results of specific heat measurements on single crystals of the frustrated quasi-2D spin-1/2 antiferromagnet Cs_2CuCl_4 (T_N=0.595 K) in external magnetic fields B30 mK. Decreasing B from high fields leads to the closure of the field-induced gap in the magnon spectrum at a critical field B_c = 8.51 T and a magnetic phase transition is clearly seen below B_c. In the vicinity to B_c, the phase transition boundary is well described by the power-law T_c(B)\propto (B_c-B)^{1/��} with the measured critical exponent ��\simeq 1.5. These findings are interpreted as a Bose-Einstein condensation of magnons., 5 pages, 4 figures, experiment and theory

The nature of the pairing states of superconducting LaNiC_{2} and LaNiGa_{2} has to date remained a puzzling question. Broken time reversal symmetry has been observed in both compounds and a group theoretical analysis implies a nonunitary triplet pairing state. However, all the allowed nonunitary triplet states have nodal gap functions but most thermodynamic and NMR measurements indicate fully gapped superconductivity in LaNiC_{2}. Here we probe the gap symmetry of LaNiGa_{2} by measuring the London penetration depth, specific heat, and upper critical field. These measurements demonstrate two-gap nodeless superconductivity in LaNiGa_{2}, suggesting that this is a common feature of both compounds. These results allow us to propose a novel triplet superconducting state, where the pairing occurs between electrons of the same spin, but on different orbitals. In this case the superconducting wave function has a triplet spin component but isotropic even parity gap symmetry, yet the overall wave function remains antisymmetric under particle exchange. This model leads to a nodeless two-gap superconducting state which breaks time reversal symmetry, and therefore accounts well for the seemingly contradictory experimental results.

This article overviews the development of heavy-fermion superconductivity, notably in such rare-earth-based intermetallic compounds which behave as Kondo-lattice systems. Heavy-fermion superconductivity is of unconventional nature in the sense that it is not mediated by electron-phonon coupling. Rather, in most cases the attractive interaction between charge carriers is apparently magnetic in origin. Fluctuations associated with an antiferromagnetic (AF) quantum critical point (QCP) play a major role. The first heavy-fermion superconductor CeCu2Si2 turned out to be the prototype of a larger group of materials for which the underlying, often pressure-induced, AF QCP is likely to be of a three-dimensional (3D) spin-density-wave (SDW) variety. For UBe13, the second heavy-fermion superconductor, a magnetic-field-induced 3D SDW QCP inside the superconducting phase can be conjectured. Such a 'conventional', itinerant QCP can be well understood within Landau's paradigm of order-parameter fluctuations. In contrast, the low-temperature normal-state properties of a few heavy-fermion superconductors are at odds with the Landau framework. They are characterized by an 'unconventional', local QCP which may be considered a zero-temperature 4 f-orbital selective Mott transition. Here, as concluded for YbRh2Si2, the breakdown of the Kondo effect concurring with the AF instability gives rise to an abrupt change of the Fermi surface. Very recently, superconductivity was discovered for this compound at ultra-low temperatures. Therefore, YbRh2Si2 along with CeRhIn5 under pressure provide a natural link between the large group of about fifty low-temperature heavy-fermion superconductors and other families of unconventional superconductors with substantially higher T c, e.g. the doped Mott insulators of the perovskite-type cuprates and the organic charge-transfer salts.

Heavy fermion metals provide a prototype setting to study quantum criticality. Experimentally, quantum critical points have been identified and studied in a growing list of heavy fermion compounds. Theoretically, Kondo destruction has provided a means to characterize a class of unconventional quantum critical points that goes beyond the Landau framework of order-parameter fluctuations. Among the prominent evidence for such local quantum criticality have been measurements in YbRh$_2$Si$_2$. A rapid crossover is observed at finite temperatures in the isothermal field dependence of the Hall coefficient and other transport and thermodynamic quantities, which specifies a $T^*(B)$ line in the temperature ($T$)-magnetic field ($B$) phase diagram. Here, we discuss what happens when temperature is raised, by analyzing the ratio of the crossover width to the crossover position. With this ratio approaching unity at $T \gtrsim 0.5$ K, YbRh$_2$Si$_2$ at zero magnetic field belongs to the quantum-critical fluctuation regime, where the single-particle spectral function has significant spectral weight at both the small and large Fermi surfaces. This implies that, in this temperature range, any measurements sensitive to the Fermi surface will also see a significant spectral weight at the large Fermi surface. The angle-resolved photoemission spectroscopy (ARPES) experiments recently reported for YbRh$_2$Si$_2$ at $T>1$ K are consistent with this expectation, and therefore support the association of the $T^*(B)$ line with the physics of Kondo destruction., submitted to proceedings of ICM15

$^{71}\mathrm{Ga}$ NQR, magnetization, and specific-heat measurements have been performed on polycrystalline Ge-doped ${\mathrm{FeGa}}_{3}$ samples. A crossover from an insulator to a correlated local moment metal in the low-doping regime and the evolution of itinerant ferromagnet upon further doping is found. For the nearly critical concentration at the threshold of ferromagnetic order, ${x}_{C}=0.15, ^{71}(1/{T}_{1}T)$ exhibits a pronounced ${T}^{\ensuremath{-}4/3}$ power law over two orders of magnitude in temperature, which indicates three-dimensional quantum critical ferromagnetic fluctuations. Furthermore, for the ordered $x=0.2$ sample (${T}_{C}\ensuremath{\approx}6$ K), $^{71}(1/{T}_{1}T)$ could be fitted well in the frame of Moriya's self-consistent renormalization theory for weakly ferromagnetic systems with $1/{T}_{1}T\ensuremath{\sim}\ensuremath{\chi}$. In contrast to this, the low-doping regime nicely displays local moment behavior where $1/{T}_{1}T\ensuremath{\sim}{\ensuremath{\chi}}^{2}$ is valid. For $T\ensuremath{\rightarrow}0$, the Sommerfeld ratio $\ensuremath{\gamma}=(C/T)$ is enhanced $(70\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{mole}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{2}$ for $x=0.1)$, which indicates the formation of heavy $3d$ electrons.

The order parameter and pairing mechanism for superconductivity in heavy-fermion compounds are still poorly understood. Scanning tunneling microscopy and spectroscopy at ultralow temperatures can yield important information about the superconducting order parameter and the gap structure. Here, we study the first heavy-fermion superconductor, ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$. Our data show the superconducting gap which is not fully formed and exhibits features that point to a multigap order parameter. Spatial mapping of the zero-bias conductance in magnetic field reveals the vortex lattice, which allows us to unequivocally link the observed conductance gap to superconductivity in ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$. The vortex lattice is found to be predominantly triangular with distortions at fields close to $\ensuremath{\sim}0.7{H}_{c2}$.

The "failed Kondo insulator" CeNiSn has long been suspected to be a nodal metal, with a node in the hybridization matrix elements. Here we carry out a series of Nernst effect experiments to delineate whether the severely anisotropic magnetotransport coefficients do indeed derive from a nodal metal or can simply be explained by a highly anisotropic Fermi surface. Our experiments reveal that despite an almost 20-fold anisotropy in the Hall conductivity, the large Nernst signal is isotropic. Taken in conjunction with the magnetotransport anisotropy, these results provide strong support for an isotropic Fermi surface with a large anisotropy in quasiparticle mass derived from a nodal hybridization., Comment: 4.5 pages, 4 figures

Type-I clathrate phase Ba(8)Ni(x)□(y)Si(46-x-y) (□ = vacancy) was obtained from the elements at 1000 °C with the homogeneity range 2.4 ≤ x ≤ 3.8 and 0 ≤ y ≤ 0.9. In addition, samples with low Ni content (x = 1.4 and 1.6; y = 0) and small Ba deficiency were prepared from the melt by steel-quenching. Compositions were established by microprobe analysis and crystal structure determination. Ba(8-δ)Ni(x)□(y)Si(46-x-y) crystallizes in the space group Pm ̅3n (No. 223) with lattice parameter ranging from a = 10.3088(1) Å for Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) to a = 10.2896(1) Å for Ba(8.00(3))Ni(3.82(4))Si(41.33(6)). Single-crystal X-ray diffraction data together with microprobe analysis indicate an increasing number of framework vacancies toward compositions with higher Ni content. For all compositions investigated, Ni K-edge X-ray absorption spectroscopy measurements showed an electronic state close to that of elemental Ni. All samples exhibit metallic-like behavior with moderate thermopower and low thermal conductivity in the temperature range 300-773 K. Samples with compositions Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) and Ba(7.9(1))Ni(1.6(1))Si(44.4(1)) are superconducting with T(c) values of 6.0 and 5.5 K, respectively.

The superconducting pairing state of LaPt$_{4}$Ge$_{12}$ is studied by measuring the magnetic penetration depth $\lambda(T,B)$ and the superfluid density $\rho_s(T)$ using a tunnel-diode-oscillator (TDO)-based method and by transverse field muon-spin rotation ($\mu$SR) spectroscopy. $\lambda(T)$ follows an exponential-type temperature dependence at $T\ll T_{c}$, but its zero-temperature value $\lambda(0)$ increases linearly with magnetic field. Detailed analyses demonstrate that both $\lambda(T)$ and the corresponding $\rho_{s}(T)$, measured in the Meissner state by the TDO method are well described by a two-gap $\gamma$ model with gap sizes of $\Delta_1(0)=1.31k_{B}T_c$ and $\Delta_2(0)=1.80k_{B}T_c$ and a very weak interband coupling. In contrast, $\rho_s(T)$, derived from the $\mu \rm{SR}$ data taken in a small field, can be fitted by a single-gap BCS model with a gap close to $\Delta_2(0)$. We conclude that LaPt$_{4}$Ge$_{12}$ is a marginal two-gap superconductor and the small gap $\Delta_1$ seems to be destroyed by a small magnetic field. In comparison, in PrPt$_4$Ge$_{12}$ the $4f$-electrons may enhance the interband coupling and, therefore, give rise to more robust multiband superconductivity., Comment: 5 pages, 4 figures

FeSb 2 films were produced in a low-pressure Ar environment by pulsed laser deposition at 355 nm. The influence of growth parameters such as substrate temperature, Ar pressure and deposition time on the growth of FeSb 2 films was studied. Nearly phase-pure FeSb 2 films with thicknesses of 100–400 nm were produced at 425 °C with an Ar pressure of 1.5–2 Pa. Thermal transport and Hall measurements were performed to explore the thermoelectric transport properties of the FeSb 2 films. A maximum thermopower of ∼120 μVK −1 at 40 K was obtained. In general it is highly important to understand the growth properties of FeSb 2 films if they are to eventually reach thermoelectric applications at cryogenic temperatures.

We review magnetic quantum-critical points (QPCs) in heavy-fermion compounds separating at zero temperature: an antiferromagnetically ordered state and a nonordered ground state. At the magnetic instability, the Fermi-liquid (FL) description valid for normal metals breaks down, giving rise to unusual, non-Fermi-liquid (NFL) low-temperature behavior. After a short introduction to phase transitions and to T = 0 phase transitions in general as well as to the physics of heavy-fermion systems, the two main theoretical scenarios describing the physics at QPCs in these systems are presented, the conventional spin-density-wave (SDW) scenario and the unconventional Kondo-breakdown scenario. Whereas for the conventional scenario experimental data for CeCu2Si2 and Ce1−xLaxRu2Si2 are discussed only briefly, we focus in more detail on the unusual behavior of CeCu6−xAux and YbRh2Si2 at their respective QPCs and show that these systems are best described within the unconventional scenario.

The origin of unconventional superconductivity, including high-temperature and heavy-fermion superconductivity, is still a matter of controversy. Spin excitations instead of phonons are thought to be responsible for the formation of Cooper pairs. Using inelastic neutron scattering, we present the first in-depth study of the magnetic excitation spectrum in momentum and energy space in the superconducting and the normal states of CeCu2Si2. A clear spin excitation gap is observed in the superconducting state. We determine a lowering of the magnetic exchange energy in the superconducting state, in an amount considerably larger than the superconducting condensation energy. Our findings identify the antiferromagnetic excitations as the major driving force for superconducting pairing in this prototypical heavy-fermion compound located near an antiferromagnetic quantum critical point., Comment: 30 pages, including supplementary material

Single crystals of HfAs(1.7)Se(0.2) are grown by chemical transport reaction and their chemical composition characterized in detail by various analytical methods. Chemical analyses and crystal structure investigations by single-crystal X-ray diffraction as well as powder diffraction with synchrotron radiation reveal a tetragonal PbFCl structure type with strong disorder caused by a significant arsenic deficiency (As(0.9)) on the 2a site and mixed occupancy of the 2c site (As(0.8)Se(0.2)). HfAs(1.7)Se(0.2) is a diamagnetic metal which transforms into a superconducting state at T(c)=0.52 K. Similar to other PbFCl-type arsenide selenides, the title compound displays a magnetic-field-independent -AT(1/2) term in the low-temperature electrical resistivity. This unusual term presumably originates from the electron scattering of structural two-level systems. According to the experimental results, HfAs1.7Se0.2 appears to be a rare example of a nonmagnetic Kondo material.

At any pressure sensitive quantum critical point (QCP) the thermal expansion is more singular than the specific heat leading to a divergence of the Gruneisen parameter. For a magnetic field sensitive QCP, the complementary property is the magnetic Gruneisen ratio which equals the magnetocaloric effect. Here we use both properties to investigate magnetic QCPs in different heavy fermion (HF) metals starting from CeNi2Ge2. The influence of dimensionality on quantum criticality is addressed by the comparison of cubic CeIn3−xSnx with layered CeMIn5−xSnx (M = Co, Rh) systems, in which Sn doping both acts as tuning parameter and introduces slight disorder. Near the field-tuned QCP in undoped CeCoIn5 a crossover scale T⋆ is discovered which separates 2D (at T>T⋆) from 3D (at T

Large single crystals of ZrAs x Se y ( x > y , x + y ≤2, PbFCl type of structure, space group P 4/ nmm ) were grown by Chemical Transport. Structural details were studied by single crystal neutron diffraction techniques at various temperatures. One single crystal specimen with chemical composition ZrAs 1.595(3) Se 0.393(1) was studied at ambient temperature ( R 1=5.10 %, wR 2=13.18 %), and a second crystal with composition ZrAs 1.420(3) Se 0.560(1) was investigated at 25 K ( R 1=2.70%, wR 2=5.70 %) and 2.3 K ( R 1=2.30 %, wR 2=4.70 %), respectively. The chemical compositions of the crystals under investigation were determined by wavelength dispersive X-ray spectroscopy. The quantification of trace elements was carried out by Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry. According to the crystal structure refinements the crystallographic 2 a site is occupied by As, together with a significant amount of vacancies. One of the 2 c sites is fully occupied by As and Se (random distribution). With respect to the fractional coordinates of the atoms, the crystal structure determinations based on the data obtained at 25.0 K and 2.3 K did not show significant deviations from ambient temperature results. The temperature dependence of the displacement parameters indicates a static displacement of As on the 2 a sites (located on the (0 0 1) planes) for all temperatures. No indications for any occupation of interstitial sites or the presence of vacancies on the Zr (2 a ) site were found.

The Hall effect of LuRh2Si2--the non-magnetic homologue of the heavy-fermion material YbRh2Si2--is studied with two different setups: In the conventional single-field geometry, the field dependence is analyzed in terms of the differential Hall coefficient. Beyond that, the recently developed crossed-field experiment allows to examine the linear-response Hall coefficient as a function of magnetic field. The results reveal the expected analogy between both experiments which corroborates the equivalent findings in YbRh2Si2. This emphasizes the applicability to investigate field-induced quantum critical points with both methods.

High-quality single crystals of the heavy fermion superconductors CeCoIn5 and CeIrIn5 have been studied by means of low-temperature scanning tunneling microscopy. Methods were established to facilitate in situ sample cleaving. Spectroscopy in CeCoIn5 reveals a gap which persists to above Tc, possibly evidencing a precursor state to superconductivity. Atomically resolved topographs show a rearrangement of the atoms at the crystal surface. This modification at the surface might influence the surface properties as detected by tunneling spectroscopy. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We present the field and temperature behavior of the narrow Electron Spin Resonance (ESR) response in YbRh2Si2 well below the single ion Kondo temperature. The ESR g factor reflects a Kondo-like field and temperature evolution of the Yb3+ magnetism. Measurements towards low temperatures (>0.5K) have shown distinct crossover anomalies of the ESR parameters upon approaching the regime of a well defined heavy Fermi liquid. Comparison with the field dependence of specific heat and electrical resistivity reveal that the ESR parameters can be related to quasiparticle mass and cross section and, hence, contain inherent heavy electron properties.

We carried out electrical resistivity experiments on (Yb,La)Rh 2 Si 2 and on Yb(Rh,Ir) 2 Si 2 under pressure and in magnetic fields. YbRh 2 Si 2 exhibits a weak antiferromagnetic transition at atmospheric pressure with a Neel temperature of only T N ≈ 70 mK. By applying a small magnetic field T N can be continuously suppressed to T = 0 at B c = 60 mT (B ⊥ c) driving the system to a quantum critical point (QCP). On applying external pressure the magnetic phase is stabilized and T N (p) is increasing as usually observed in Yb-based heavy-fermion metals. Substituting Yb by La or Rh by Ir allows to create a negative chemical pressure, La (Ir) being smaller than Yb (Rh), and eventually to drive YbRh 2 Si 2 to a pressure controlled QCP. In this paper we compare the effect of external hydrostatic pressure and chemical substitution on the ground-state properties of YbRh 2 Si 2 .

The single phase clathrate-I Ba8Ge43□3 (space group Iad (no. 230), a = 21.307(1) A) was synthesized by quenching the melt between cold steel plates. Specimens for physical property measurements were characterized by microstructure analysis and X-ray diffraction on polycrystalline samples as well as single crystals. Transport properties including thermopower, electrical resistivity, thermal conductivity and specific heat were investigated in a temperature range of 2–673 K. The electrical resistivity exhibits a metal-like temperature dependence below 300 K turning into a semiconductor-like behaviour above 300 K. The analysis of the specific heat at low temperature indicates a finite density of states at the Fermi level, thus corroborating the metallic character below 300 K. The temperature dependence of the specific heat was modelled assuming Einstein-like localized vibrations of Ba atoms inside the cages of the Ge framework. A conventional crystal-like behaviour of the thermal conductivity with a low lattice contribution (κl(300 K) = 2.7 W m−1 K−1) has been evidenced.

The clathrate I Ba8Ge43□3 [space group $$ Ia\bar{3}d $$ , no. 230, a = 21.307(1) A] has been synthesized as a single phase and characterized by x-ray powder diffraction and metallographic analysis. Electrical and thermal transport measurements have been performed in the temperature range of 5 K to 673 K. Ba8Ge43□3 displays the electrical resistivity of a poor metal at low temperatures, with semiconducting-like behavior appearing above 300 K.

We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancelation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh2Si2 in terms of their dependence on temperature and magnetic field., Comment: 5pages, 1 Figure

We report low-temperature measurements of the electrical resistivity of Yb ( Rh 1 - x Co x ) 2 Si 2 single crystals with 0 ≤ x ≤ 0.12 . The isoelectronic substitution of Co on the Rh site leads to a decrease of the unit cell volume which stabilizes the antiferromagnetism. Consequently, the antiferromagnetic transition temperature increases upon Co substitution. For x = 0.07 Co content a subsequent low-temperature transition is observed in agreement with previous susceptibility measurements and results on YbRh 2 Si 2 under hydrostatic pressure. Above the Neel transition the resistivity follows a linear temperature dependence of a non-Fermi liquid similar to that of YbRh 2 Si 2 .

The new example of a ferromagnetic Kondo lattice, CeRuPO, is a good candidate which offers the opportunity to investigate the physical properties near a ferromagnetic instability. Macroscopic experiments evidenced a Kondo temperature T K = 10 K and Curie temperature of T C = 14 K . We have investigated the pressure–temperature phase diagram of CeRuPO by means of electrical resistivity measurements on high quality single crystals in the temperature interval from 1.8 to 300 K. The pressure dependence of the ordering temperature follows the anticipated behavior of a Ce-based Kondo lattice system, where pressure is expected to suppress the magnetic order. The critical pressure for the suppression of the transition temperature to zero is estimated to be p c ≈ 3.2 GPa .

We present an overview of magnetotransport measurements on the heavy-fermion superconductor CeIrIn5. Sensitive measurements of the Hall effect and magnetoresistance are used to elucidate the low-temperature phase diagram of this system. The normal-state magnetotransport is highly anomalous, and experimental signatures of a pseudogap-like precursor state to superconductivity, as well as evidence for two distinct scattering times governing the Hall effect and the MR, are observed. Our observations point out the influence of antiferromagnetic fluctuations on the magnetotransport in this class of materials. The implications of these findings, both in the context of unconventional superconductivity in heavy-fermion systems and in relation to the high-temperature superconducting cuprates, are discussed.

We report on the magnetic and electrical properties of Pr0.6Sr0.4MnO3 single crystals. This compound undergoes a continuous paramagnetic-ferromagnetic transition with a Curie temperature T C∼301 K and a first-order structural transition at T S∼64 K. At T S, the magnetic susceptibility exhibits an abrupt jump, and a corresponding small hump is seen in the resistivity. The critical behavior of the static magnetization and the temperature dependence of the resistivity are consistent with the behavior expected for a nearly isotropic ferromagnet with short-range exchange belonging to the Heisenberg universality class. The magnetization (M–H) curves below T S are anomalous in that the virgin curve lies outside the subsequent M–H loops. The hysteretic structural transition at T S as well as the irreversible magnetization processes below T S can be explained by phase separation between a high-temperature orthorhombic and a low-temperature monoclinic ferromagnetic phase.

A variety of unconventional pairing mechanisms seem to be operating in heavy-fermion superconductors. We address briefly Cooper pairing mediated by magnetic excitons in UPd 2 Al 3 and by valence fluctuations in pressurized CeCu 2 Si 2 . Moreover, evidence from inelastic neutron scattering for HF superconductivity being mediated by quantum critical spin-density-wave fluctuations in CeCu 2 Si 2 is discussed.

We report on inelastic neutron scattering experiments on the heavy-fermion superconductor CeCu2Si2 to study the magnetic excitation spectrum in single crystals. Elastic short-range magnetic correlations have been observed in the superconducting state. While the low-energy magnetic response seems to be quasielastic in the normal state above T c ≈ 600 mK , a truly inelastic response is detected in the superconducting state well below T c . This inelastic response is discussed in connection to the properties of the superconducting state.

Quantum criticality describes the collective fluctuations of matter undergoing a second-order phase transition at zero temperature. Heavy fermion metals have in recent years emerged as prototypical systems to study quantum critical points. There have been considerable efforts, both experimental and theoretical, which use these magnetic systems to address problems that are central to the broad understanding of strongly correlated quantum matter. Here, we summarize some of the basic issues, including i) the extent to which the quantum criticality in heavy fermion metals goes beyond the standard theory of order-parameter fluctuations, ii) the nature of the Kondo effect in the quantum critical regime, iii) the non-Fermi liquid phenomena that accompany quantum criticality, and iv) the interplay between quantum criticality and unconventional superconductivity., (v2) 39 pages, 8 figures; shortened per the editorial mandate; to appear in Nature Physics. (v1) 43 pages, 8 figures; Non-technical review article, intended for general readers; the discussion part contains more specialized topics

CeCu 2 Si 2 plays a crucial role in studying heavy fermion superconductivity and related phenomena. Both the superconducting and normal state properties support the existence of two quantum phase transitions in pressurized CeCu 2 (Si 1− x Ge x ) 2 . Superconductivity in the low-pressure region occurs around an antiferromagnetic quantum critical point, indicating magnetically mediated superconductivity. A novel superconducting state, likely paired via valence fluctuations, develops around a first-order isostructural volume-collapse transition at high pressures while crossing from the heavy fermion state to the intermediate valence state. In this paper, we briefly review the recent status of CeCu 2 Si 2 , in particular its unique properties under pressure.

In this paper, a brief survey is given on a number of research activities devoted to exploring the relationship between superconductivity and magnetism in f-electron systems. The starting point for these activities has been the pioneering work of 1958 by Matthias and coworkers illustrating the antagonistic nature of the two phenomena. Subsequent efforts concerned the investigation of Kondo superconductors and Kondo-lattice systems (in the 1970s), heavy-fermion metals (in the 1980s and 90s) and quantum critical materials (in the last decade). The latter systems are especially interesting as they promise a deeper insight into the mutual interplay between unconventional superconductivity and magnetism.

Single crystals of the PbFCl-type arsenide selenides ThAsSe and ZrAs1.40 Se0.50 display intrinsic structural disorder, as inferred from X-ray and electron diffraction as well as transmission-electron-microscopy studies. This structural disorder is held responsible for the formation of tunneling states that are a cause of glass-type temperature dependencies of both the thermal conductivity and the specific heat in the millikelvin temperature range. The electrical resistivity increases as –AT 1/2 upon cooling to below 16 K. Furthermore, it is unaffected by the application of a magnetic field. These observations are considered as evidence for an experimental realization of a two-channel Kondo effect derived from structural two-level systems.

The heavy-fermion superconductor CeCoIn5 is the first material, where different experimental probes show strong evidence pointing to the realization of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The inhomogeneous superconducting FFLO state with a periodically modulated order parameter was predicted to appear in Pauli-limited, sufficiently clean type-II superconductors already more than 40 years ago. On the other hand, CeCoIn5 is supposed to be close to a magnetic quantum critical point (QCP) showing strong antiferromagnetic (AFM) spin fluctuations (SF) at atmospheric pressure. We studied the evolution of the FFLO phase away from the influence of the strong AFM-SF by heat capacity experiments under pressure (0 GPa

The heavy-fermion superconductor CeCoIn 5 is the first material, where different experimental probes show strong evidence pointing to the realization of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state. The inhomogeneous superconducting FFLO state with a periodically modulated order parameter was predicted to appear in Pauli-limited, sufficiently clean type-II superconductors already more than 40 years ago. On the other hand, CeCoIn 5 is supposed to be close to a magnetic quantum critical point (QCP) showing strong antiferromagnetic (AFM) spin fluctuations (SF) at atmospheric pressure. We studied the evolution of the FFLO phase away from the influence of the strong AFM-SF by heat capacity experiments under pressure (0 GPa ≤ P ≤1.5 GPa, 0 T ≤µ 0 H ≤14 T, and 100 mK ≤ T ≤4 K). Our results prove the stability of the the FFLO phase under pressure. It even expands, while the Pauli-limiting becomes weaker and the AFM-SF are suppressed. This shows the intriguing influence of the AFM-SF on the FFLO state.

We report specific heat, magnetocaloric effect and magnetization measurements on single crystals of the frustrated quasi-2D spin -½ antiferromagnet Cs2CuCl4 in the external magnetic field 0≤B≤12 T along a-axis and in the temperature range 0.03 K≤T≤6 K. Decreasing the applied magnetic field B from high fields leads to the closure of the field induced gap in the magnon spectrum at a critical field Bc8.44 T and a long-range incommensurate state below Bc. In the vicinity of Bc, the phase transition boundary is well described by the power law TN~(Bc-B)1/ with the measured critical exponent 1.5. These findings provide experimental evidence that the scaling law of the transition temperature TN can be described by the universality class of 3D Bose–Einstein condensation (BEC) of magnons.

We report on inelastic neutron scattering experiments on YbRh 2 Si 2 powder to study the low-energy spin dynamics at temperatures between T = 0.8 and 22 K. The low-energy magnetic response is quasielastic. However, it exhibits an unusual form not modelled by a simple relaxation rate yielding a Lorentzian lineshape, but can satisfactorily be described by a phenomenological model involving a distribution of relaxation rates. The lower bound of the relaxation rates varies roughly linear with temperature indicating a pronounced slowing down of the critical modes above the antiferromagnetic ordering temperature T N ≈ 70 mK .

This work deals with the preparation and characterization of tungsten tips for the use in UHV low-temperature scanning tunneling microscopy and spectroscopy (STM and STS, respectively). These specific environments require in situ facilities for tip conditioning, for further sharpening of the tips, as well as for reliable tip characterization. The implemented conditioning methods include direct resistive annealing, annealing by electron bombardment, and self-sputtering with noble gas ions. Moreover, results from in situ tip characterization by field emission and STM experiments were compared to ex situ scanning electron microscopy. Using the so-prepared tips, high resolution STM images and tunneling spectra were obtained in a temperature range from ambient down to 350 mK, partially with applied magnetic field, on a variety of materials.

We present measurements of the thermopower $S(T)$ on ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}$ between 2 K and 300 K. For low Rh content, the system behaves as a ferromagnetic Kondo system with a Curie temperature ${T}_{\mathrm{C}}$ of about 6 K, a Kondo scale smaller than ${T}_{\mathrm{C}}$, and an overall crystal electric field splitting of 210 K. As the Rh content increases ${T}_{\mathrm{C}}$ is suppressed, while the average Kondo scale gets larger. Simultaneously, the presence of different Ce environments leads to a broad distribution of local Kondo scales ranging from very small values to above 50 K. As a consequence, large thermopower values are observed over an extended temperature range. Close to the critical concentration we find power-law dependencies of $S/T$ vs $T$ down to 2 K. For a Rh content of $x=0.95$ we may show explicitly that the thermopower contains contributions from Ce sites with different local energy scales.

The Nernst effect and thermopower of the prototypical Yb-based intermediate valence compound YbAl(3) were investigated. Different to the thermopower whose absolute values are enhanced with increasing temperature and assume a broad maximum at 175 K, the Nernst coefficient of YbAl(3) is enhanced only below T ≈ 75 K. While the two quantities in the heavy-fermion compound CeCu(2)Si(2) were recently found to be related by the anomalous Hall mobility due to the local asymmetric Kondo scattering, this theorem fails when being applied to YbAl(3). Rather, the thermopower of YbAl(3) is well described by a simple narrow-band model. We discuss the reason for this in terms of the intermediate valence nature of YbAl(3) that is conceptually different from the local Kondo physics.